email servers generally aren't publicly accessible, rather they're only accessible to authorized (registered) users

Yes, sending email requires authorization to the SMTP server but MTA to MTA communications (as in when your mailserver actually sends your email to the recipients mail server) are clear text and can easily be intercepted.

The difference between telephone calls and email is that you generally don't have access to the things you need to listen in on a telephone call, but email you only need to have access to one of the routers that it's routed through